Combustion instability has been a continuing problem in the design of low-emission, high performing combustion chambers for gas turbines, boilers, heaters, furnaces, and other devices. Combustion instability is generally understood as high amplitude pressure oscillations that occur within the combustion chamber due to the turbulent nature of the combustion process and large volumetric energy release within the closed cavity of the combustion chamber. Many factors may contribute to a stable or an unstable state within the combustion chamber, including the fuel content, fuel and/or air injection speed or inlet pressure, fuel/air concentration, temperature changes within the combustion chamber, and/or the stability of the flame. Operating instabilities may further be amplified by the physical mechanisms of a particular combustion system design. Unfortunately, combustion instability diminishes engine system performance, and the vibrations resulting from pressure oscillations can potentially cause severe damage to hardware components, including the combustion chamber.
Conventional approaches for correcting combustion instability have involved passive control methods, such as changing the design of the combustor and/or revising the operating conditions. Examples of passive control are modification of the fuel injection distribution pattern, and changing the shape or capacity of the combustion chamber. Passive controls are often very costly and place unacceptable limits on combustor performance.
Recently, active controls have been developed to modulate certain aspects of the combustor environment to counteract the variable heat release within the combustion chamber that leads to an oscillating condition. Active controls may modify the pressure within the system and/or regulate the fuel or air flow into the system in response to detected unstable conditions. For example, a common method of active control is fuel or air metering, involving monitoring the stability of the combustion chamber, detecting and characterizing the instability, and cycling the flow of fuel or air injected into the chamber at the same frequency which produces the undesired oscillation, but at a phase angle such that the imposed modulation cancels, or effectively suppresses, the undesired oscillation. The fuel or air modulation is designed to counteract the oscillation in each cycle, and, therefore, relatively high frequency actuators are necessary. Disadvantages of active control incorporating high frequency modulation are the necessity of high frequency actuators and a detailed understanding of the actuator effect.
Controlling combustion instability is of increasing concern, as current attention is directed to high efficiency combustion systems that have very low exhaust emission levels, including emissions of NO.sub.x and CO pollutants. These systems generally involve lean premixing (LPM) strategies, wherein the fuel and air are thoroughly mixed prior to combustion, such that the resulting concentration of the fuel and air mixture minimizes the generation of pollutants upon combustion. This lean fuel and air concentration is approximately half the stoichiometric concentration required for combustion (i.e. self-supporting reactions), and therefore, combustion instability may cause even greater problems in LPM systems than in combustion systems operating at the stoichiometric fuel/air concentration. For example, as the fuel/air concentration approaches the stoichiometric limit for sustained combustion (the lean blow-out boundary), the variation of combustion temperature with fuel/air concentration becomes much greater, even to the point of extinguishing the flame. In addition, small changes in fuel/air concentration may result in large fluctuations, or oscillations, in temperature and pressure.
A need continues in the art for a low-cost, easily installed method and apparatus for actively controlling combustion instability.
The present periodic equivalence ratio modulation (PERM) method and apparatus is a unique approach for actively controlling oscillations within a combustion chamber. More specifically, the method involves periodically modulating the equivalence ratio for a combustion device, such that the combustion device operates in alternate stable conditions, or in alternate stable and unstable conditions. The periodic modulation is achieved by producing cycles of low frequency pulses of fuel applied over, or in addition to, the main fuel line control, which determines the fuel flow rate through the combustion system, and, therefore, the desired time-average equivalence ratio. In this way, the periodic equivalence ratio modulation (PERM) technique maintains the desired time-average equivalence ratio, while effectively controlling the pressure oscillations within the combustion chamber to eliminate or significantly reduce combustion instability.
Therefore, in view of the above, a basic object of the present invention is to provide a low-cost, easily installed method and apparatus for actively controlling combustion instability.
A further object of this invention is to provide active control for combustion instability that utilizes commercially available hardware.
Another object of this invention is to provide active control for combustion instability that is installed outside of the combustion chamber components, i.e. the engine pressure casing on a gas turbine.
Yet another object of this invention is to provide active control for combustion instability that is applicable to LPM systems.
Yet another object of this invention is to provide active control for combustion instability that is not limited to operation at the acoustic frequency of the combustor.
Yet another object of this invention is to provide active control for combustion instability that reduces pollutant emissions.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.